1. Field of the Invention
The present invention relates to a load control device for controlling the amount of power delivered to an electrical load, specifically, to an electronic dimming ballast having advanced control of a power converter.
2. Description of the Related Art
Electronic ballasts for fluorescent lamps typically can be analyzed as comprising a “front-end” and a “back-end”. The front-end often includes a rectifier for receiving an alternating-current (AC) mains line voltage and producing a rectified voltage VRECT, and a boost converter for receiving the rectified voltage VRECT and generating a direct-current (DC) bus voltage VBUS across a bus capacitor. The boost converter is an active circuit for boosting the magnitude of the DC bus voltage above the peak of the line voltage and for improving the total harmonic distortion (THD) and the power factor of the input current to the ballast. The ballast back-end typically includes a switching inverter circuit for converting the DC bus voltage VBUS to a high-frequency AC square-wave voltage VSQ, and a resonant tank circuit for generating a sinusoidal voltage VSIN from the square-wave voltage VSQ and coupling the sinusoidal voltage VSIN to the lamp electrodes of the fluorescent lamp. The amount of power delivered to the lamp may be adjusted by controlling a duty cycle DCSQ of the square-wave voltage VSQ to thus control the intensity of the lamp from a low-end intensity LLE to a high-end intensity LHE.
The boost converters of most prior art ballasts have controlled the magnitude of the bus voltage VBUS to a constant magnitude independent of the operating conditions of the ballast. Some prior art ballasts have been operable to turn off the boost converter (such that the magnitude of the DC bus voltage equals approximately the peak magnitude of the line voltage) when the intensity of the lamp is near the low-end intensity LLE, as described in commonly-assigned U.S. Pat. No. 7,075,254, issued Jul. 11, 2006, entitled LIGHTING BALLAST HAVING BOOST CONVERTER WITH ON/OFF CONTROL AND METHOD OF BALLAST OPERATION, the entire disclosure of which is hereby incorporated by reference. However, the prior art ballast of U.S. Pat. No. 7,075,254 was only able to control the boost converter to two discrete states (i.e., on and off).
Thus, there is a need for an electronic dimming ballast that is able to control the magnitude of the bus voltage VBUS according to a more advanced control scheme during the different modes of operation of the ballast.